The present invention relates to a device using an electron beam, and more particularly, the present invention relates to electron beam interferomatic technology using an electron microscope.
An electron interferometer is used for measuring the phase change of the electron beam as an approach that quantitatively measures the magnetic field of a substance or in vacuum. FIG. 1A shows an optical setup for interferometry for a generally employed electron holography. The electron beam 2 emitted from the electron source 1 advances as shown in the drawing. Current density is adjusted by the first irradiation electron lens 3 and the second irradiation electron lens 4. The specimen 6 is placed at one side of the optical axis between the second irradiation electron lens 4 and the objective lens 5 so that the specimen 6 on the objective plane 42 of the objective lens is irradiated with the electron beam. The image formed by action of the objective lens is enlarged by a magnifying lens 9. The electron beam 7 passing through the specimen in the first region on the objective plane and the electron beam 8 passing through the second region on the objective plane are curved inward by the second electron biprism 10, and are superposed on the observation plane 11. The interference fringes are then detected through interference. The phase reconstruction is performed in reference to the obtained interference fringes so as to obtain the phase change of the electron beam through the specimen 6.
The electron biprism has the function of deflecting the electron beam passing to the left or right of the electrode filament between the parallel plates in parallel with the electron beam advancing direction inward or outward with respect to the optical axis in the electric field between the filament electrode and the parallel flat plate, which is generated by applying electrical potential to the electrode filament. Generally, the system equipped with the mechanism that rotates with respect to the optical axis, and the mechanism that moves the electron biprism in the plane perpendicular to the optical axis is commercially available.
In the aforementioned method, the electron beam 7 passing through the first region and the electron beam 8 passing through the second region are adjacent with each other at the specimen position. The width of the interference region converted in terms of the specimen plane is restricted by the coherence distance of the electron beam radiated to the specimen plane in the specimen in-plane direction (see Non Patent Literature 1, Tonomura A. (1987) Applications of electron holography, Rev. Mod. Phys. 59: pp. 639-669). Patent Literatures of Japanese Patent Application Laid-Open Publication Nos. 2006-164861 and 2011-249191 relate to the aforementioned device.
The coherence distance of the aforementioned electron interferometer will be described referring to FIG. 2. The structure shown in FIG. 2 satisfies the following formula (1) having an opening angle set to β, the wavelength of the electron beam set to λ, and the coherence distance set to Lc relative to the setup shown in FIG. 2 including the electron source 101, the size 102 of the electron source, the distance 103 between the electron source 101 and the specimen plane 104, and the specimen plane 104.[Formula 1]Lc=λ/2β  (1)
Assuming that the coherence distance is set to Lc, the maximum possible width Wmax of the interference region will satisfy the following formula.[Formula 2]Wmax<Lc  (2)
As for the actual electron beam device, the image of the electron source is equivalent to the image reduced or enlarged by action of the irradiation lens system, objective lens system, or image forming lens system. The objective plane is also equivalent to the image plane of the specimen 6, which is enlarged or reduced by the lens of the irradiation lens system, the objective lens system or the image forming lens system at a position different from the position of the specimen on the electron beam optical axis.
Restriction of the coherence distance of the irradiation electron beam on the specimen plane may deteriorate contrast of the interference fringes as the width of the interference region is increased for observing inside of the thin film specimen. Therefore, it is not possible to make the interference width wider than the coherence distance. For that reason, the generally employed electron holography allows observation of the high contrast interference fringes only in the narrow interference region.
Charging caused by irradiating the specimen with converged electron beam influences the electron beam 8 passing through the second region, and the interference fringes are distorted. There has been a problem of difficulty in obtaining the image with reconstructed phase with high accuracy. Besides, the specimen with large leakage magnetic field also contributes to the problem. The leak magnetic field influences the electron beam 8 passing through the second region, and the interference fringes are distorted, thus interfering with the phase analysis with high accuracy. In order to solve the aforementioned problems, it is necessary to increase the in-plane distance between the electron beam 7 transmitting through the specimen in the first region and the electron beam 8 passing through the second region on the specimen plane.
Japanese Patent Application Laid-Open Publication No. 2006-164861 proposes the scan interference electron microscope as the method of irradiating the specimen plane at different regions with electron beams split by the electron biprism provided in the irradiation system without using the objective lens for forming the specimen image. This method includes the steps of irradiating the vacuum region on the specimen plane, and the specimen with the convergence electron probe, allowing the detector at lower side to detect interference fringes of the electron beam transmitting through the vacuum and the one transmitting through the specimen, and scanning the probe or the specimen while obtaining the phase information so as to obtain the electromagnetic field information in the plane of the specimen. This method allows easy acquisition of data once the condition is determined, easy change in magnification, and high ratio of signal of the phase information to noise. Meanwhile, in the case where the specimen is irradiated with the convergence electron beam, and the specimen has a certain thickness for the purpose of obtaining all the electromagnetic information data about the region at the observation points during scanning, which is irradiated with the cone-like electron probe, resolution at the position as the upper surface or the lower surface of the specimen upon passage of the cone-like electron probe through the specimen may be increased corresponding to the larger diameter of the electron beam. For example, likewise tomography, the method is not suitable for application to the approach requiring the image with two-dimensional projection phase information that is not blurred in the lateral direction.
The inventor has proposed the method of irradiating the specimen with the electron beam 7 transmitting through the specimen 6 in the first region on the specimen plane and the electron beam 8 passing through the second region, while keeping a distance therebetween as shown in FIG. 1B in Japanese Patent Application Laid-Open Publication No. 2011-249191. This method includes the steps of splitting the electron wave by the first electron biprism 12 provided in the irradiation system, deflecting the advancing direction, irradiating the specimen with the electron beam 7 transmitting through the first region (specimen) on the specimen plane and the electron beam 8 passing through the second region while keeping the distance therebetween, deflecting the electron beam by the second electron biprism 10 downstream of the specimen in the electron beam advancing direction, and interfering on the observation plane 11 to detect the interference fringes. The electron beam coherent in the in-plane direction of the electron beam 7 transmitting through the first region (specimen) on the specimen plane and the electron beam 8 passing through the second region is split and radiated. Although the distance between the electron beam 7 transmitting through the first region (specimen) on the specimen plane and the electron beam 8 passing through the second region is made longer than the coherence distance of the irradiating electron beam in the in-plane direction without splitting the electron beam by the first biprism 12, it is possible to obtain the high contrast interference fringes in principle.
However, the actual electron beam device needs to optimize the setup of the irradiation system and the condition for using the lens current of the irradiation system for interference of the electron beam on the specimen plane so as to obtain the effect of the aforementioned split irradiation. Specifically, there has been a problem that the region of the specimen plane on the electron optics where the electron beam interference occurs is hidden by the electrode filament of the first electron biprism, and the interference fringes cannot be obtained.